Technology providing an insulating mat between a catalyst carrier heated by electrical conduction and a case containing the carrier of the said catalyst is known (For example, refer to Patent Reference 1). It is possible to inhibit the flow of electricity by means of this mat in the case where electricity is conducted through the catalyst carrier.
However, because moisture is contained in engine exhaust gasses, water sometimes condenses in the case or the like. This water flows inside the case, attaches to the mat and is then absorbed in the mat. Water absorbed in the mat migrates to the center of the mat, and then, because the water in the mat is vaporized by the heat of the exhaust gas and the heat of the heating element, it is removed over time. However, if the engine is started and stopped repeatedly within a relatively short time, the quantity of condensed water increases, and removal of the water within the mat becomes difficult. Because the humidity in the vicinity of the electrodes increases as a result, and the insulation resistance between the electrodes and the case decreases, there is the concern that electricity will flow from the electrodes to the case.